A wide variety of devices such as Liquid Crystal Displays (LCDs), smart phones, tablet computers utilize flat glass sheets. A preferred technique for manufacturing these flat glass sheets is the fusion process. In the fusion process, the glass sheets are made by using glass manufacturing vessels that contain precious metals, e.g. platinum or platinum alloys. The precious metals are generally considered to be inert in relation to most glasses, and thus should not cause any inclusions in the glass sheets. However, this is not necessarily valid.
There are oxidation reactions that occur at the metal/glass interface inside the glass manufacturing vessel which leads to the generation of gaseous inclusions in the molten glass and thus the glass sheet. One of the more common oxidation reactions that occur at the metal/glass interface is the conversion of negatively charged oxygen ions to molecular oxygen which is caused by the thermal breakdown of water and hydroxyl species in the molten glass. This phenomenon occurs because at the elevated temperatures of glass melting and delivery, a low partial pressure of hydrogen exists in the molten glass. Thus, when hydrogen comes in contact with the precious metal vessel containing the molten glass, the hydrogen rapidly permeates out of the glass manufacturing vessel, depleting the metal/glass interface of hydrogen. Based on the chemical balance, for every mole of hydrogen that leaves the glass manufacturing vessel, ½ mole of oxygen is left behind at the glass/metal interface. Thus, as hydrogen leaves the glass manufacturing vessel, the oxygen level or partial pressure of oxygen at the metal/glass interface increases, which leads to the generation of blisters or gaseous inclusions in the molten glass. In addition, there are other reactions which involve the catalyzing or oxidation of other species within the molten glass such as halogens (Cl, F, Br) which can lead to the generation of gaseous inclusions within the molten glass and the resulting glass sheet. Further, there are oxidation reactions which can occur due to electrochemical reactions at the metal/glass interface. These electrochemical reactions can be associated with thermal cells, galvanic cells, high AC or DC current applications and grounding situations.
Today, there are several known methods that can be used to address these problematical oxidation reactions which cause the formation of gaseous inclusions in the molten glass and the resulting glass sheet. These known methods range from the use of glass coatings, atmospheric control around the external surfaces of the glass manufacturing vessels to DC protection. All of these methods have their uses, but come with significant costs and can be difficult to implement and maintain. For instance, there is a method which involves the use of a humidity controlled enclosure that surrounds one or more of the precious metal-containing glass manufacturing vessels and is used to control the partial pressure of hydrogen outside the vessel(s) so as to reduce the formation of gaseous inclusions in the glass sheets. Several different types of these humidity controlled enclosures are discussed in U.S. Pat. No. 5,785,726 and U.S. Pat. No. 7,628,039 (the contents of which are incorporated by reference herein). Although the use of a humidity controlled enclosure is effective, it is also expensive both in the capital cost to construct as well as the cost of operation. The principle expenses of operation are nitrogen, energy for air conditioning and steam production as well as the energy required for the fans that drive gas circulation. Thus, it would be desirable to provide an alternative method to prevent the formation of gaseous inclusions in glass sheets. This need and other needs are satisfied by the present invention.